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MS HVAC practice questions

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Question 1HVACR Core Knowledge and Skills

A technician measures a supply-air temperature rise across an electric resistance heater. Which relationship correctly expresses the sensible heat added to the airstream?

  1. Q = 500 × CFM × ΔT
  2. Q = 1.08 × CFM × ΔT Correct answer.
  3. Q = 1.08 × CFM ÷ ΔT
  4. Q = 4.5 × CFM × ΔT

Explanation

The sensible heat equation for air uses the constant 1.08 when airflow is in CFM and temperature difference is in °F, giving Btu/h. The constant 4.5 applies to total heat with enthalpy difference, and 500 applies to water in gallons per minute. Dividing by ΔT would invert the relationship and is dimensionally wrong.

Question 2HVACR Core Knowledge and Skills

On a split system, the liquid line is warm at the condensing unit but the suction line at the evaporator is frosted. Which condition most directly explains frosting at the evaporator outlet?

  1. Refrigerant undercharge
  2. High condenser subcooling
  3. Oversized liquid line
  4. Low evaporator airflow Correct answer.

Explanation

Restricted airflow reduces the heat available to boil refrigerant, so evaporating temperature and pressure drop and the coil falls below freezing. Frost then forms where saturated refrigerant is coldest, often near the outlet. Charge and line sizing affect superheat and pressure drop but do not by themselves create a uniformly frosted coil with normal airflow.

Question 3HVACR Core Knowledge and Skills

Which instrument is the correct choice for verifying the superheat of a refrigerant blend at the evaporator outlet?

  1. Sling psychrometer
  2. Clamp meter
  3. Vacuum micron gauge
  4. Manifold gauge set with a temperature probe Correct answer.

Explanation

Superheat is the difference between the actual suction-line temperature and the saturated evaporating temperature corresponding to the suction pressure. A manifold gauge set with a temperature probe provides both readings directly. A psychrometer measures air conditions, a clamp meter measures current, and a micron gauge measures vacuum depth.

Question 4HVACR Core Knowledge and Skills

A compressor nameplate lists RLA 22.0 A and LRA 120 A. A technician reads 26 A on the common leg while the unit runs. What does this reading most likely indicate?

  1. Locked rotor current
  2. Normal running current within tolerance
  3. An open run winding
  4. Current above rated load, suggesting a developing problem Correct answer.

Explanation

RLA is the maximum continuous running current the manufacturer rates for the compressor under rated conditions. A reading of 26 A exceeds the 22 A rating and suggests high load, low voltage, or mechanical drag. Locked rotor current would be near the LRA value, and an open winding would draw no current.

Question 5HVACR Core Knowledge and Skills

Which safety practice applies before brazing refrigerant lines on a system that previously held R-410A?

  1. Vent refrigerant to atmosphere to clear the lines
  2. Open the circuit and braze while the system is pressurized
  3. Purge with oxygen to displace refrigerant
  4. Recover refrigerant and break the vacuum with dry nitrogen Correct answer.

Explanation

Refrigerant must be recovered before opening a circuit, and the lines should be purged or flowed with dry nitrogen to prevent oxidation and exclude air. Brazing a pressurized system is unsafe, and venting refrigerant violates refrigerant-handling rules. Oxygen must never be used because it supports combustion and can form explosive compounds with oil.

Question 6HVACR Core Knowledge and Skills

A technician is evacuating a system and the micron gauge stalls near 1500 microns after a long run, then slowly rises after the pump is isolated. What is the most probable cause?

  1. Overcharged system
  2. Moisture or a leak in the system Correct answer.
  3. Excessively deep vacuum
  4. Faulty micron gauge calibration only

Explanation

A vacuum that stalls well above the target and rises once the pump is valved off indicates vapor is entering or being generated inside the system. Moisture boiling off or a small leak both produce this pattern. A properly tight, dry system holds a low micron reading after isolation.

Question 7HVACR Core Knowledge and Skills

Which condition defines the dew point of a refrigerant blend such as R-407C during evaporation?

  1. The temperature at which the first bubble of vapor forms
  2. The temperature at which liquid and vapor have equal mass
  3. The temperature at which the blend stops absorbing heat
  4. The temperature at which the last drop of liquid boils off Correct answer.

Explanation

A zeotropic blend changes temperature as it evaporates, so it has a temperature glide. The bubble point is where the first vapor forms, while the dew point is where the last liquid boils away. Superheat for such blends is normally referenced to the dew point at the suction pressure.

Question 8HVACR Core Knowledge and Skills

A belt-driven blower delivers adequate airflow but the belt squeals and shows glazing. Which correction is most appropriate?

  1. Increase motor pulley diameter
  2. Tighten the belt to maximum tension
  3. Replace the belt and align the pulleys Correct answer.
  4. Reduce blower speed setting

Explanation

Squealing and glazing indicate the belt is slipping, usually from misalignment, wear, or incorrect tension. The correct repair is to replace the glazed belt and align the sheaves, then set tension per the manufacturer. Over-tightening damages bearings, and changing pulley size alters airflow rather than fixing the slip.

Question 9HVACR Core Knowledge and Skills

Which statement correctly describes the function of the reversing valve in a heat pump?

  1. It bypasses the accumulator during defrost
  2. It meters refrigerant into the evaporator
  3. It changes the direction of refrigerant flow to swap the roles of the indoor and outdoor coils Correct answer.
  4. It regulates the compressor discharge temperature

Explanation

The reversing valve is a four-way valve that redirects discharge and suction gas so the indoor coil can act as either the evaporator or the condenser. In heating mode the outdoor coil becomes the evaporator, and in cooling mode the indoor coil does. Metering and temperature regulation are handled by other components.

Question 10HVACR Core Knowledge and Skills

A heat pump in heating mode ices heavily on the outdoor coil and the defrost cycle never initiates. Which sensor or control is the first suspect?

  1. Liquid-line filter drier
  2. Outdoor coil temperature sensor and defrost board Correct answer.
  3. Reversing valve solenoid
  4. Indoor thermostat anticipator

Explanation

Defrost initiation depends on the outdoor coil temperature sensor and the defrost control board, often combined with a time-temperature strategy. If the sensor reads incorrectly or the board fails, the unit will not enter defrost. A stuck reversing valve or restricted drier would produce different symptoms.

Question 11HVACR Core Knowledge and Skills

A technician must select a replacement capacitor for a permanent split capacitor (PSC) blower motor. Which rating combination must match the original?

  1. Voltage only
  2. Case diameter only
  3. Microfarad rating and voltage rating Correct answer.
  4. Frequency only

Explanation

A run capacitor must match the original microfarad value closely to preserve motor torque and speed, and its voltage rating must equal or exceed the original. Case size and shape matter for mounting but not electrically. Frequency is set by the power supply, not the capacitor.

Question 12HVACR Core Knowledge and Skills

A technician reads 0 V across the contacts of a contactor while the coil is energized, but the compressor does not start. What is the next logical check?

  1. Replace the contactor coil
  2. Confirm line voltage is present at the contactor line terminals Correct answer.
  3. Adjust the thermostat heat anticipator
  4. Test the capacitor microfarad value only

Explanation

Zero volts across closed contacts means both sides are at the same potential, which could be line voltage present or no voltage at all. Before condemning the contactor, verify line-side voltage is actually reaching the contactor. If line voltage is present and passes through to the load, the fault lies downstream.

Question 13HVACR Core Knowledge and Skills

Which type of expansion device maintains a constant superheat at the evaporator outlet over a range of loads?

  1. Fixed orifice piston
  2. Capillary tube
  3. Thermostatic expansion valve Correct answer.
  4. Hand valve

Explanation

A thermostatic expansion valve uses a sensing bulb to modulate refrigerant flow and hold superheat near a set value as load changes. Fixed orifices and capillary tubes are fixed-restriction devices that cannot adjust to load. A hand valve requires manual adjustment and is not automatic.

Question 14HVACR Core Knowledge and Skills

A system shows high suction pressure, low superheat, and a sweating suction line at the compressor. Which condition fits these readings?

  1. Overcharge of refrigerant Correct answer.
  2. Low indoor airflow
  3. Undercharge of refrigerant
  4. Restricted liquid line

Explanation

High suction pressure with low superheat and a cold, sweating suction line suggests excess refrigerant returning to the compressor, consistent with overcharge. Undercharge produces high superheat and low suction pressure, while a liquid-line restriction causes low suction pressure and high superheat. Low airflow would raise superheat and lower suction pressure.

Question 15HVACR Core Knowledge and Skills

Which duct design principle best reduces excessive static pressure in a residential supply system?

  1. Using flexible duct with sharp bends
  2. Sizing ducts to maintain low velocity and minimizing fittings Correct answer.
  3. Reducing the return grille free area
  4. Increasing the number of 90-degree elbows

Explanation

Static pressure loss rises with velocity and with the number of fittings, especially sharp bends. Proper duct sizing keeps velocity within design limits, and minimizing fittings reduces turbulence and pressure drop. Adding elbows, sharp bends, and restrictive grilles all increase resistance.

Question 16HVACR Core Knowledge and Skills

A technician is verifying combustion safety on a gas furnace. Which measurement is most important for confirming complete combustion?

  1. Return-air temperature
  2. Blower motor amp draw
  3. Condensate pH
  4. Flue-gas carbon monoxide and oxygen concentrations Correct answer.

Explanation

Combustion analysis measures oxygen, carbon monoxide, and carbon dioxide in the flue gas to confirm complete combustion and proper excess air. High CO with low oxygen indicates incomplete combustion and possible unsafe operation. Motor amps, return-air temperature, and condensate pH do not assess combustion quality.

Question 17HVACR Core Knowledge and Skills

A technician finds a cracked heat exchanger in a gas furnace. What is the appropriate action?

  1. Continue operation and monitor CO levels
  2. Increase blower speed to dilute flue gases
  3. Seal the crack with high-temperature tape
  4. Red-tag the unit and shut off the gas supply Correct answer.

Explanation

A cracked heat exchanger can allow flue gases, including carbon monoxide, to enter the supply air. The unit must be taken out of service and the gas supply shut off, with the customer notified and the unit tagged. Taping, continued operation, or dilution does not correct the hazard.

Question 18HVACR Core Knowledge and Skills

Which pressure reading should a technician expect at the suction service valve of a properly charged R-410A system operating with a 40°F evaporating temperature?

  1. About 250 psig
  2. About 68 psig
  3. About 118 psig Correct answer.
  4. About 400 psig

Explanation

R-410A has a saturation pressure near 118 psig at 40°F. The technician should compare the measured suction pressure to a pressure-temperature chart for the specific refrigerant. Values near 68 psig correspond to R-22 at 40°F, and 250 psig and above are condenser-range pressures.

Question 19HVACR Core Knowledge and Skills

What is the primary purpose of a crankcase heater on a compressor?

  1. To prevent refrigerant migration and oil dilution during the off cycle Correct answer.
  2. To defrost the outdoor coil
  3. To raise discharge pressure during start-up
  4. To warm the suction gas before compression

Explanation

A crankcase heater keeps the compressor oil warmer than the coldest part of the system during the off cycle, discouraging refrigerant from condensing in the crankcase and diluting the oil. It does not heat suction gas, raise discharge pressure, or perform defrost. Preventing oil dilution protects lubrication at start-up.

Question 20HVACR Core Knowledge and Skills

A technician measures a 20°F temperature difference across the evaporator coil with a clean filter and normal airflow. The system uses a TXV. Which conclusion is most reasonable?

  1. The coil is frozen solid
  2. The compressor has failed
  3. The system is severely overcharged
  4. The system is operating within a typical acceptable range Correct answer.

Explanation

A temperature difference of roughly 16°F to 22°F across a properly operating evaporator is generally acceptable, depending on humidity and design. A 20°F split with clean airflow suggests normal operation. Overcharge, compressor failure, and a frozen coil would produce other symptoms such as abnormal pressures or low airflow.

Question 21HVACR Core Knowledge and Skills

Which refrigerant handling practice is required when a technician opens a system containing a regulated refrigerant?

  1. Vent only the vapor portion to atmosphere
  2. Recover refrigerant into approved recovery equipment Correct answer.
  3. Burn off the charge with a torch
  4. Release the charge slowly to avoid oil spray

Explanation

Regulated refrigerants must be recovered using certified recovery equipment and cannot be knowingly vented to the atmosphere. Slow release, partial venting, and burning are all prohibited and unsafe. Recovery also prevents contamination and allows the refrigerant to be reclaimed or recycled.

Question 22HVACR Core Knowledge and Skills

A compressor trips its internal overload after running for several minutes on a hot day. Which combination of readings would most support a diagnosis of high head pressure causing the trip?

  1. Low discharge pressure and low subcooling
  2. High discharge pressure and high subcooling with normal suction pressure Correct answer.
  3. Normal pressures with high superheat
  4. High suction pressure and low discharge pressure

Explanation

High discharge pressure with elevated subcooling points to a condenser that cannot reject heat, such as a dirty coil or failed fan, which raises compressor load and can trip the overload. Low discharge pressure suggests undercharge or low load. Normal pressures with high superheat point to a metering or charge issue instead.

Question 23HVACR Core Knowledge and Skills

Which device protects a compressor from operating at excessively low suction pressure?

  1. Defrost thermostat
  2. Float switch
  3. Low-pressure switch Correct answer.
  4. High-pressure switch

Explanation

A low-pressure switch opens the control circuit when suction pressure falls below a setpoint, protecting the compressor from loss of charge or low-load operation. A high-pressure switch protects against excessive discharge pressure. Defrost thermostats and float switches serve other functions.

Question 24HVACR Core Knowledge and Skills

A technician is troubleshooting a system where the indoor fan runs continuously but the outdoor unit never starts. The thermostat is calling for cooling and the contactor coil reads 0 V. Which component is the most likely cause?

  1. Open low-voltage control circuit Correct answer.
  2. Restricted filter drier
  3. Defective run capacitor
  4. Failed condenser fan motor

Explanation

If the thermostat is calling but the contactor coil has no voltage, the low-voltage control circuit is open somewhere between the transformer and the coil. Possible causes include a tripped safety, broken wire, or failed control board. A capacitor, drier restriction, or fan motor failure would not remove coil voltage.

Question 25HVACR Core Knowledge and Skills

Which statement about the relationship between airflow and sensible capacity in a direct-expansion cooling coil is correct?

  1. Reducing airflow increases both sensible and latent capacity
  2. Airflow has no effect on coil capacity
  3. Reducing airflow lowers sensible capacity and increases the sensible heat ratio shift toward latent removal Correct answer.
  4. Increasing airflow raises the coil temperature and increases latent removal

Explanation

Lower airflow reduces the amount of air passing over the coil, lowering sensible capacity while the colder coil removes more moisture, so latent capacity rises relative to sensible. Higher airflow warms the coil and reduces dehumidification. Airflow therefore strongly affects both total capacity and the sensible-to-latent balance.

Question 26HVACR Core Knowledge and Skills

A technician is brazing a copper suction line on a system that uses a nitrogen purge. What is the primary reason for flowing nitrogen through the tubing during the braze?

  1. To carry flux into the joint for a stronger bond
  2. To cool the copper so the joint does not anneal
  3. To increase the working pressure so leaks show up sooner
  4. To prevent internal oxidation (scale) from forming inside the tubing Correct answer.

Explanation

Copper heated in air forms oxide scale on the inside of the tubing, and that scale later breaks loose and migrates to the compressor, metering device, and filter-drier. A slow nitrogen purge displaces oxygen so the internal surface stays bright and clean. Nitrogen does not act as a flux and is not used to pressurize the joint during brazing.

Question 27HVACR Core Knowledge and Skills

A technician needs to recover refrigerant from a residential split system before replacing the outdoor unit. Which practice satisfies the requirement to recover refrigerant rather than vent it?

  1. Using certified recovery equipment and storing the refrigerant in a DOT-approved recovery cylinder Correct answer.
  2. Pumping the charge into the indoor coil and leaving it there under pressure
  3. Venting the charge slowly through the gauge manifold to avoid slugging the compressor
  4. Releasing the charge into the airstream of the condenser fan so it disperses quickly

Explanation

Refrigerant must be recovered with certified equipment and placed in a properly rated recovery cylinder, never released to the atmosphere. Recovery cylinders are DOT-approved and are not filled above the marked fill limit. Venting, dispersing, or leaving charge trapped in a component are not acceptable substitutes for recovery.

Question 28HVACR Core Knowledge and Skills

A technician is checking a compressor's electrical circuit and measures resistance between the common terminal and the run terminal. What is this measurement primarily used to determine?

  1. Whether the start capacitor is internally shorted
  2. Whether the crankcase heater is drawing current
  3. Whether the contactor coil is energized
  4. Whether the run winding is continuous and within the expected resistance range Correct answer.

Explanation

Resistance measured across a motor winding indicates continuity and approximate winding condition; an open winding reads infinite resistance and a shorted winding reads near zero. Capacitors, contactor coils, and heaters are separate components tested on their own circuits. Winding resistance readings are compared with expected values for the specific motor.

Question 29HVACR Core Knowledge and Skills

A technician is preparing to leak-test a newly installed refrigerant system. Which method is appropriate for locating a small refrigerant leak on an operating system?

  1. A vacuum gauge held below 500 microns for ten minutes
  2. An ohmmeter placed across the liquid line and suction line
  3. A soap-bubble solution applied to the compressor crankcase only
  4. An electronic refrigerant leak detector traced along joints and fittings Correct answer.

Explanation

An electronic leak detector senses halogenated refrigerant escaping at joints, fittings, and brazed connections and is the standard tool for pinpointing small leaks. Soap bubbles work but must be applied to every suspect joint, not just the crankcase. A micron gauge and an ohmmeter do not detect refrigerant leaks.

Question 30HVACR Core Knowledge and Skills

A technician measures the temperature of the liquid line at the outlet of the condenser and compares it with the condensing temperature derived from the discharge pressure. What does the difference represent?

  1. Subcooling, which indicates how much liquid refrigerant has been cooled below condensing temperature Correct answer.
  2. The evaporator temperature difference, which indicates airflow across the indoor coil
  3. Superheat, which indicates how much vapor has been heated above evaporating temperature
  4. The compressor discharge superheat, which indicates motor winding temperature

Explanation

Subcooling is condensing temperature minus liquid-line temperature, and it confirms that the condenser has produced solid liquid at the end of the coil. Superheat is measured at the evaporator outlet, not the condenser outlet. Evaporator temperature difference and discharge superheat are different measurements taken elsewhere in the system.

Question 31HVACR Core Knowledge and Skills

A technician is setting the charge on a system with a thermostatic expansion valve using the superheat method. Which condition should the technician target at the evaporator outlet?

  1. Zero superheat so the coil is fully flooded under all conditions
  2. A superheat value within the manufacturer's specified range for the current indoor wet-bulb and outdoor conditions Correct answer.
  3. A superheat equal to the condenser subcooling reading
  4. The highest achievable superheat so no liquid can reach the compressor

Explanation

Superheat is set to the manufacturer's target for the measured indoor wet-bulb and outdoor dry-bulb conditions, because the correct value shifts with load. Zero superheat risks liquid floodback to the compressor. Excessively high superheat starves the coil and reduces capacity. Superheat and subcooling are independent measurements and are not set equal.

Question 32HVACR Core Knowledge and Skills

A technician is diagnosing a PSC condenser fan motor that hums but does not rotate until the blade is nudged. Which component is the most likely cause?

  1. The run capacitor has failed or lost capacitance Correct answer.
  2. The contactor coil has an open winding
  3. The defrost control board has failed
  4. The low-pressure switch is stuck closed

Explanation

A PSC motor depends on its run capacitor to create the phase shift needed for starting torque; a weak or failed capacitor leaves the motor humming without rotation. A failed contactor coil or defrost board would prevent the motor from receiving power at all. A stuck-closed low-pressure switch would not cause a hum-and-nudge symptom.

Question 33HVACR Core Knowledge and Skills

A technician is checking a gas furnace and observes a steady flame that lifts off the burner ports and appears to burn above the burner surface. Which condition does this most likely indicate?

  1. Excessive primary air or excessive manifold gas pressure Correct answer.
  2. A failed hot surface igniter that is still warm
  3. A blocked flue that is forcing products back through the burner
  4. Insufficient primary air at the burner

Explanation

A flame that lifts or blows off the ports usually means too much primary air or too high a manifold pressure, so the flame speed cannot hold it at the burner. Insufficient primary air produces a lazy, yellow, sooting flame instead. A blocked flue and a failed igniter do not produce flame lift.

Question 34HVACR Core Knowledge and Skills

A technician is checking the temperature rise of a gas furnace. The data plate specifies a rise of 35°F to 65°F, and the measured rise is 80°F. Which condition is most consistent with this reading?

  1. The return-air temperature is higher than the supply-air temperature
  2. Airflow across the heat exchanger is lower than the design airflow Correct answer.
  3. The gas manifold pressure is set too low
  4. The blower is running at a higher speed than specified

Explanation

Temperature rise equals supply-air temperature minus return-air temperature, and a rise above the rated range means the same heat is being delivered to too little air. Restricted filters, a dirty evaporator coil, or a blower set too slow all reduce airflow and raise the rise. Low manifold pressure would reduce the rise, and a faster blower would also reduce it.

Question 35HVACR Core Knowledge and Skills

A technician is checking a heat pump in cooling mode and finds the outdoor fan running, the compressor running, and the indoor blower running, but the supply air is only slightly cooler than the return air. Which additional measurement best separates a refrigerant charge problem from an airflow problem?

  1. The resistance of the indoor blower motor windings
  2. The evaporator superheat and subcooling compared with target values Correct answer.
  3. The voltage at the defrost control board
  4. The current draw of the outdoor fan motor

Explanation

Superheat and subcooling reveal how the refrigerant circuit is behaving relative to its targets, which distinguishes an undercharge or restriction from an airflow deficit. Blower winding resistance, defrost board voltage, and fan motor current do not characterize the refrigeration cycle. Airflow should also be verified, but the refrigerant measurements are what separate the two possibilities.

Question 36HVACR Core Knowledge and Skills

A technician is measuring airflow with a duct traverse using a pitot tube and finds the velocity pressure is very low across the duct. Which interpretation is correct?

  1. Velocity pressure is a static measurement and does not indicate airflow
  2. Low velocity pressure corresponds to high air velocity because the two are inversely related
  3. Velocity pressure below 0.1 in. w.c. indicates the pitot tube is clogged and the reading is invalid
  4. Low velocity pressure corresponds to low air velocity, so airflow through that duct is low Correct answer.

Explanation

Velocity pressure is the difference between total pressure and static pressure, and it rises with the square of air velocity. A very low velocity pressure therefore indicates low velocity and low airflow at that traverse point. Velocity pressure is not inversely related to velocity and is not a static-only measurement.

Question 37HVACR Core Knowledge and Skills

A technician is evaluating a refrigerant blend that has a measurable temperature glide during evaporation. Which statement about charging such a system is correct?

  1. Charge as a liquid to avoid fractionation, and evaluate superheat using the dew-point pressure-temperature relationship Correct answer.
  2. Charge as a vapor, and evaluate subcooling using the dew-point relationship
  3. Charge as a vapor to avoid slugging, and evaluate superheat using the bubble-point relationship
  4. Charge as a liquid, and evaluate superheat using the bubble-point relationship

Explanation

Blends with glide must be charged as liquid so the components do not separate, and because the evaporator performance relates to the dew point, superheat is evaluated with the dew-point column. The bubble point applies to the start of boiling and is used differently, such as for subcooling on the liquid side. Vapor charging risks fractionation and an incorrect mixture in the cylinder.

Question 38HVACR Core Knowledge and Skills

A technician is reviewing the pressure-enthalpy diagram of a refrigeration cycle and notes that the expansion device is passing liquid with no change in enthalpy. Which statement best explains this behavior?

  1. The expansion process is isothermal, so temperature stays constant while pressure drops
  2. The expansion device compresses the refrigerant, raising both pressure and enthalpy
  3. The expansion device adds heat to the refrigerant, which raises enthalpy across the valve
  4. The expansion process is modeled as adiabatic and isenthalpic, so enthalpy stays constant while pressure and temperature drop Correct answer.

Explanation

A thermostatic or fixed orifice expansion device is treated as an adiabatic throttling process in which no work or heat transfer occurs, so enthalpy is essentially constant across it. Pressure falls and part of the liquid flashes to vapor, which lowers the mixture temperature. The process neither adds heat, holds temperature constant, nor compresses the refrigerant.

Question 39HVACR Core Knowledge and Skills

A technician is commissioning a variable-speed air handler and observes that the blower ramps up and down as the filter loads. Which control strategy explains this behavior?

  1. A motor with an open winding that is cycling on thermal overload
  2. A fixed-speed PSC motor with a failed run capacitor
  3. A direct-drive motor wired to line voltage with no control module
  4. Constant-torque or constant-static control that adjusts speed to maintain a target, which can mask a loading filter Correct answer.

Explanation

Many variable-speed blowers use constant-torque or constant-static control, adjusting speed to hold a target as static pressure changes, so a loading filter can be partially compensated by higher speed. A failed PSC capacitor or an open winding would prevent normal operation rather than produce smooth ramping. A line-voltage direct-drive motor has no speed control at all.

Question 40HVACR Core Knowledge and Skills

A technician is evaluating a compressor that has a suction-line accumulator installed. Which function does the accumulator serve in the refrigeration circuit?

  1. It acts as a receiver to hold the system charge during service
  2. It traps and meters liquid refrigerant back to the compressor as vapor to prevent slugging Correct answer.
  3. It separates oil from the discharge gas before the condenser
  4. It stores excess liquid refrigerant for the condenser during low-load operation

Explanation

A suction accumulator is located between the evaporator and the compressor, where it collects any liquid that leaves the evaporator and allows it to boil off so only vapor returns to the compressor. It is not a condenser-side receiver and does not separate oil from discharge gas. Oil separators and receivers perform those different jobs.

Question 41Refrigeration Systems and Components

In a vapor-compression refrigeration cycle, which component's primary function is to reject heat from the refrigerant to the surrounding air or water?

  1. The expansion valve
  2. The condenser Correct answer.
  3. The evaporator
  4. The compressor

Explanation

The condenser receives high-pressure, high-temperature vapor from the compressor and rejects heat so the refrigerant condenses to a liquid. The evaporator absorbs heat instead, the expansion valve drops pressure, and the compressor raises pressure and temperature.

Question 42Refrigeration Systems and Components

What is the primary purpose of superheating refrigerant vapor leaving the evaporator?

  1. To reduce the refrigerant mass flow rate
  2. To increase the condenser's subcooling margin
  3. To ensure no liquid droplets reach the compressor Correct answer.
  4. To lower the compressor discharge temperature

Explanation

A small amount of superheat guarantees the refrigerant is fully vaporized before entering the compressor, protecting it from liquid slugging. Superheat has no direct role in subcooling, and it actually raises discharge temperature rather than lowering it.

Question 43Refrigeration Systems and Components

Which refrigerant classification is defined by the ASHRAE safety group designation A1?

  1. Higher toxicity and no flame propagation
  2. Lower toxicity and higher flammability
  3. Lower toxicity and no flame propagation Correct answer.
  4. Higher toxicity and higher flammability

Explanation

ASHRAE Standard 34 assigns a letter for toxicity (A lower, B higher) and a number for flammability (1 none, 2 lower, 3 higher). A1 therefore indicates lower toxicity with no flame propagation, typical of refrigerants such as R-134a.

Question 44Refrigeration Systems and Components

A technician measures the suction line temperature at 45°F and the evaporator saturation temperature at 35°F. What is the superheat?

  1. 45°F
  2. 35°F
  3. 10°F Correct answer.
  4. 80°F

Explanation

Superheat is the difference between the actual suction line temperature and the saturation temperature at the same pressure. Subtracting 35°F from 45°F gives 10°F of superheat, which is a typical target for a properly charged system.

Question 45Refrigeration Systems and Components

Which component in a refrigeration circuit meters refrigerant flow and separates the high-pressure and low-pressure sides?

  1. The muffler
  2. The accumulator
  3. The receiver
  4. The expansion valve Correct answer.

Explanation

The expansion valve throttles refrigerant from condensing pressure to evaporating pressure, creating the pressure boundary between the high and low sides. The accumulator, receiver, and muffler serve other functions and do not meter flow.

Question 46Refrigeration Systems and Components

What does a thermostatic expansion valve (TXV) use as its controlling signal?

  1. The compressor discharge temperature
  2. The ambient air temperature at the condenser
  3. The temperature and pressure at the evaporator outlet Correct answer.
  4. The pressure differential across the condenser

Explanation

A TXV balances bulb pressure, evaporator pressure, and spring pressure to maintain a target superheat at the evaporator outlet. It responds to evaporator outlet conditions, not condenser or discharge conditions.

Question 47Refrigeration Systems and Components

Which statement correctly describes the pressure relationship in a properly operating vapor-compression system?

  1. The low side pressure equals atmospheric pressure at all times
  2. The low side pressure is higher than the high side pressure
  3. The high side pressure is higher than the low side pressure Correct answer.
  4. Both sides operate at equal pressure during steady operation

Explanation

The compressor maintains a pressure difference, with condensing pressure on the high side and evaporating pressure on the low side. This differential drives refrigerant flow through the metering device and back to the compressor.

Question 48Refrigeration Systems and Components

Which refrigerant handling practice is required for recovery of refrigerant from a system before opening it to atmosphere?

  1. Recover refrigerant into an approved recovery cylinder Correct answer.
  2. Release refrigerant only during daylight hours
  3. Vent refrigerant to the atmosphere if the charge is small
  4. Discharge refrigerant through the condenser fan

Explanation

Refrigerant must be recovered using certified recovery equipment into approved containers to prevent release to the atmosphere. Venting, fan discharge, and time-of-day releases are prohibited handling practices.

Question 49Refrigeration Systems and Components

A system shows low suction pressure, high superheat, and a frosted suction line at the evaporator inlet. Which condition is most consistent with these readings?

  1. Excessively subcooled liquid at the condenser outlet
  2. An overcharge of refrigerant
  3. A restricted or underfeeding expansion valve Correct answer.
  4. A failed condenser fan motor

Explanation

Low suction pressure with high superheat and frosting near the evaporator inlet indicates the evaporator is being starved of refrigerant, often from a restricted or underfeeding TXV. An overcharge or condenser fault would generally raise head pressure and reduce subcooling rather than starve the evaporator.

Question 50Refrigeration Systems and Components

Which reading would indicate that a condenser is rejecting heat effectively?

  1. A compressor discharge temperature that is unusually high
  2. A condenser subcooling value within the manufacturer's specified range Correct answer.
  3. A suction superheat above 40°F
  4. A suction pressure below the design value

Explanation

Subcooling within the specified range shows the condenser is condensing liquid properly and maintaining adequate liquid seal. High superheat, high discharge temperature, or abnormally low suction pressure suggest problems elsewhere, not effective condenser performance.